<?php

/**
 * Add the field to the checkout
 **/
// add_action('woocommerce_after_order_notes', 'my_custom_checkout_field');

// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
     $fields['billing']['naissance'] = array(
        'label'     => __('Date de naissance', 'woocommerce'),
		'placeholder'   => _x('JJ/MM/AAAA', 'placeholder', 'woocommerce'),
		'required'  => true,
		'class'     => array('form-row-wide'),
		'clear'     => true
     );

     return $fields;
}

?>